The present invention relates to IMPATT amplifiers and oscillators, and more particularly to a circuit for suppressing undesirable parametric oscillations capable of being generated in such amplifiers and oscillators.
In parametric amplifiers and oscillators, the parametric effects, under controlled conditions, provide amplification and frequency conversion. IMPATT amplifiers and oscillators, however, employ semiconductors which exhibit nonlinear reactance and parametrically generated negative resistance under large-signal conditions; these effects are usually undesirable since they cause spurious oscillations leading to severe distortions of the output signal, a reduction of power output and efficiency, and an increase in noise output.
In recent years, there have been significant improvements in avalanche diodes to realize the full high power capability of IMPATT devices operating at high frequency. Among the improvements has been the reduced series resistance of, for example, the gallium arsenide (GaAs) avalanche diodes having a Read Structure which has yielded more than 5 watts cw at greater than 30% efficiency when operated as an IMPATT oscillator. However, the limitation to the application of this diode to its operation in an IMPATT amplifier or oscillator circuit has been its inherent nonlinear reactance. When the reactance is varied at large-signal frequencies, identified as f.sub.s, unwanted negative-resistance is parametrically generated. Because of the reduced series resistance of the improved diode, the magnitude of the parametrically generated negative-resistance can readily exceed its series resistance; consequently, this nonlinear, negative-resistance diode is vulnerable to unwanted parametric oscillations induced when pumped by f.sub.s. This results in severe distortions in its output signal and a reduction of its power output and efficiency.
When the nonlinear, negative-resistance diodes had a higher inherent series resistance, there was little problem from parametrically induced idler frequencies; however, the IMPATT oscillators and amplifiers incorporating these higher resistance diodes had inherently low power output capability and efficiency. With the advent of the newer diodes where the series resistance was lower than the parametrically generated negative resistance, circuit elements have had to be added to the signal input circuit to overcome the negative-resistance which had to have negligible effect on the large-signal input to be amplified, f.sub.s, and, at the same time, would represent positive-resistance loading at the frequencies of the parametrically induced idler frequencies, f.sub.1 and f.sub.2. This has been difficult to achieve, and as a result, the IMPATT oscillators and amplifiers have been operated at reduced power in order to reduce the distortion of the output signal and, consequently, are operated at a reduced efficiency.
Suppression of unwanted parametric oscillations in oscillators and amplifiers containing IMPATT diodes is necessary in order to eliminate severe distortions in the output signals. These parametric oscillations are generated in IMPATT diodes as in other semiconductor devices that exhibit nonlinear reactance behavior under large-signal conditions. This behavior depends on the extent to which the device is being pumped or driven into its nonlinear reactance range by the large-signal input, identified as f.sub.s, as well as on circuit conditions. One commonly observed nonlinearity is the saturation of the output signal by f.sub.s due to the combined effects of the widening of the avalanche zone of the semiconductor, space charge effects and nonsaturated drift velocities. Another nonlinear effect is spurious parametric oscillations due to a negative conductance created by modulation of ionization rates in the semiconductor by the electric field across the semiconductor. These must be suppressed for high efficiency operation. The cause of these spurious oscillations is the time-varying inductance produced by the variation of the semiconductors' ionization rates.
As is well known to those skilled in the art, the relationships between the pumping signal frequency f.sub.s and the spurious oscillation frequencies f.sub.1 and f.sub.2, commonly referred to as a parametric-pair of idler frequencies, can be derived from the Manley-Rowe equations. For any single valued, lossless, nonlinear reactance in a system which is excited so that the current and voltage have frequency components of the form mf.sub.2 + nf.sub.3, where m and n are integers: ##EQU1## where P.sub.m,n is the average power flowing into the reactance at the frequencies .+-. (mf.sub.2 + mf.sub.3). Let us assume the existence of a parametric pair at the frequencies f.sub.1 and f.sub.2 caused by the pumping signal at f.sub.3 ; that is, EQU f.sub.3 = f.sub.1 + f.sub.2
or EQU f.sub.1 = f.sub.3 - f.sub.2
where f.sub.1 and f.sub.2 are the parametric-pair of idler frequencies and f.sub.3 is the pumping or driving signal f.sub.s. In regard to the relationships between the average powers flowing into the reactances at the pumping and idler frequencies, we can derive the following set of equations for this system: ##EQU2## Among the conclusions which may be drawn from equations (3) and (4) applicable to this system are:
a. The presence of a pumping source at large-signal frequency f.sub.s requires, according to our sign convention, positive P.sub.s and, consequently, negative P.sub.2 and P.sub.1. The circuit can, therefore, deliver power at f.sub.2 and f.sub.1, the idler frequencies.
b. If f.sub.1 can be prevented from delivering power, then f.sub.2 will also not deliver power; thus, if f.sub.1 is suppressed, then f.sub.2 will be simultaneously suppressed.